Gallium nitride (GaN) based blue-violet semiconductor lasers are likely to have far reaching technological and commercial effects. These semiconductor lasers emit near 400 nanometers, about half the wavelength of typical gallium arsenide (GaAs) based semiconductor lasers. The shorter wavelengths allow GaN based semiconductor lasers to achieve higher spatial resolution in applications such as optical storage and printing. Blu-ray Disc (trademark) and High Density Digital Versatile Disc (HD-DVD) (trademark) are, for example, next-generation optical disc formats that utilize blue-violet semiconductor lasers for the storage of high-definition video and data.
GaN based blue-violet semiconductor lasers typically comprise a multilayer semiconductor structure formed on a substrate (e.g., sapphire), and electrical contacts that facilitate the application of an electrical voltage to a portion of the multilayer structure. FIG. 1A shows a sectional view of a conventional GaN based semiconductor laser 100, while FIG. 1B shows the relative conduction band levels, Ec, of various constituent layers and sublayers under typical operating bias conditions. The semiconductor laser comprises a sapphire substrate 110, an n-type gallium nitride (n-GaN) base layer 120, an n-type aluminum gallium nitride (n-AlGaN) cladding layer 130 and an n-side undoped GaN waveguide layer 140. A multiple quantum well (MQW) active layer 150 is formed on top of the n-side waveguide layer. These quantum wells comprise three indium gallium nitride (InGaN) well sublayers 152 separated by GaN barrier sublayers 154. A p-type aluminum gallium nitride (p-AlGaN) electron blocking layer 160 is formed on the active layer, followed by a p-side undoped GaN waveguide layer 170 and a p-type stressed layer superlattice (SLS) cladding layer 180. The SLS cladding layer comprises alternating sublayers of p-AlGaN and p-GaN, 182 and 184, respectively.
Two electrical contacts 190, 195 are operative to allow the application of electrical voltage to the semiconductor laser 100. The applied electrical voltage causes electrons and holes to be injected into the MQW active layer 150. Some of these injected electrons and holes are trapped by the quantum wells and recombine, generating photons of light. By reflecting some of the generated light from facets formed at two opposing vertical surfaces of the semiconductor laser (not shown), some photons are made to pass through the MQW active layer several times, resulting in stimulated emission of radiation.
The waveguide layers 140, 170 form an optical film waveguide in the semiconductor laser 100 and serve as local reservoirs for electrons and holes for injection into the MQW active layer 150. The optical film waveguide, in turn, is completed by cladding layers 130, 180 which have a lower index of refraction than the waveguide layers. The cladding layers act to further restrict the generated light to the MQW active layer of the semiconductor laser.
As shown in FIG. 1B, the electron blocking layer 160 in the semiconductor laser 100 is configured to have a relatively high conduction level, Ec. The electron blocking layer, thereby, forms a potential barrier that acts to suppress the flow of electrons from the MQW active layer 150. Advantageously, this reduces the threshold current of the semiconductor laser (the minimum current at which stimulated emission occurs), allowing for a higher maximum output power. Electron blocking layers are described for use in GaAs based semiconductor lasers in, for example, U.S. Pat. No. 5,448,585 to Belenky et al., entitled “Article Comprising a Quantum Well Laser,” which is incorporated herein by reference. Nevertheless, the implementation of electron blocking layers in GaN based semiconductor lasers is problematic. Electron blocking layers located between the MQW active layer and one of the waveguide layers have been shown to cause excessive physical stress on the active layer which may, in turn, cause cracking.
As a response, attempts have been made to move the electron blocking layer away from the MQW active layer and into the p-side waveguide layer. Asano et al. in “100-mV Kink-Free Blue-Violet Laser Diodes with Low Aspect Ratio,” IEEE Journal of Quantum Electronics, Vol. 39, No. 1, January 2003, also incorporated herein by reference, for example, demonstrates the use of a p-AlGaN electron blocking layer formed in a p-side waveguide layer of a semiconductor laser similar to the semiconductor laser 100 shown in FIG. 1. Unfortunately, however, such efforts have shown limited success in reducing the physical stress in the MQW active layer. Stress induced cracking still remains an issue for GaN based semiconductor lasers.
There is, as a result, a need for a GaN based blue-violet semiconductor laser design that includes an electron blocking layer without the concomitant physical stress on the active layer.